In order to fabricate an ion optical device capable of forming a certain electric field distribution in a space, it is often necessary to machine an ion optical lens with a complex surface. By taking a two-dimensional linear ion trap as an example, its main body usually requires two pairs of quadrupole field electrodes (refer to PCT Application Publication No. WO2003/067623 A1) with a hyperbolic cylindrical surface. In order to avoid difficulty in machining the quadrupole field electrodes with the hyperbolic cylindrical surface, a manner of using a planar structure to replace a curved structure is also proposed, for example, US20040135080A1. In order to improve field perfection, a Chinese Patent with Patent No. ZL200410024946 describes a planar linear ion trap mass analyzer constructed with a printed circuit board. Such a two-dimensional linear ion trap consists of a main body portion and end caps at two ends of the main body portion.
FIG. 1 is a structural view of a typical two-dimensional linear ion trap. A slot 4 on a main body portion 2 is used for ion extraction. In use, a Direct Current (DC) voltage applied to the two end caps 1 and 3 is higher than that applied to the main body portion 2, and in addition, an RF voltage varying with time is further applied to the main body portion 2, for the purpose of generating a quadrupole filed inside the linear ion trap. When an ion (a positive ion is taken as an example) meets a quadrupole stability condition, a Radio Frequency (RF) electric field applied perpendicular to an axial direction of the ion trap and a DC electric field applied parallel to the axial direction of the ion trap are bound, and mass analysis also can be performed at this time, so as to achieve the purpose of ion storage and ion analysis.
A substrate typically used for a printed circuit is an FR4 type epoxy resin fiberboard, and the material will release various gases adsorbed in a vacuum environment, which is not suitable for a vacuum environment where the ion trap works (the vacuum degree reaches 10−1-10−4 Torr). Moreover, for such a material, processes of photolithography and corrosion of metal coatings on the surface of the printed circuit board are often used to make discrete electrodes having high symmetry. FIG. 2A to FIG. 2C illustrate three adverse consequences possibly generated through the above corrosion process. The corrosion process defines that the corrosion depth may not be very deep, which means, the grooves with depth-to-width ratio (aspect ratio) larger than or equal to one could not be easily fabricated or accurately controlled. Therefore either it may result in that the corrosion time is too long to undermine morphology of a metal electrode 5, as shown in FIG. 2A or the corrosion depth is too shallow so that an electrode player with a depth more than 0.2 mm cannot be made on the surface, which results in that a charge 6 is easily accumulated in regions between the discrete electrodes 5 during use of the device, as shown in FIG. 2B, and the accumulated charge 6 easily forms an electric field in a space other than the electrode surface, causing distortion of the quadrupole field inside the ion trap, and affecting ion trap mass analysis capability. Even if appropriate depth and width parameters can be found, as concentration of a corrosive liquid will become smaller with change of time in a region where it contacts with metal, the corrosion speed becomes slow with change of time, with the consequence that the morphology of the corroded metal electrode 5 is trapezoidal, as shown in FIG. 2C. Situations shown in FIG. 2A and FIG. 2C all may cause deterioration of position accuracy and collimation of the electrode pattern, and affect field distribution in the trap, thereby adversely affecting transmission or analysis performance of the ion optical device.
There are other methods used for fabricating isolated electrodes. In TW200733149A the author disclosed a method for preparing a kind of chip resistor, in which transverse separated lines parallel to each other could be generated one by one using separated linear cutting. These lines could be also cut the component surface into certain depth to form electrodes which may be separated as well. Using this method, separated electrodes described in patent CN1838371 could be fabricated. However, the previous cutting method can only fabricate full planar electrodes and can not realize highly accurate electrodes formation, such as less than 5 microns in tolerance. Also, the integrated error from each cutting will further enlarge the inaccuracy. In precise ion optics as mass analyzer or mobility analyzer, the insulator on the planar will cause surface charge problem, and better tolerance must be guaranteed in order to obtain excellent mass analysis performances, especially for planar quadrupole and ion trap analyzers.